Typically ultrasonic transducers are used which are capable of both emitting ultrasonic waves into a fluid and receiving ultrasonic waves. The propagation time of ultrasonic signals which are transmitted from an emitter to a receiver is influenced by the flow of the fluid. It is possible to infer the flow velocity of the fluid from the degree of influence of the propagation time.
British Published Patent Application No. 2 101 318 describes an ultrasonic flow meter in which two ultrasonic transducers are mounted on opposite sides of a pipe through which a fluid flows. The transducers are situated slightly offset with respect to one another, so that ultrasonic waves emitted by one transducer and received by the second transducer propagate at an angle to the flow direction of the fluid which is different from 90°.
In addition to the system described in British Published Patent Application No. 2 101 318 A, ultrasonic flow meters are also known in which ultrasonic waves emitted by an ultrasonic transducer are initially reflected one time or multiple times before they are received by a second ultrasonic transducer situated on the same side of the pipe through which the fluid flows as the first ultrasonic transducer. Such systems are described, for example, in European Published Patent Application No. 0 477 418, in British Published Patent Application No. 1 541 419 and in Japanese Published Patent Application No. 59100820. In European Published Patent Application No. 0 477 418 A1, a unit made up of two ultrasonic transducers and one reflector system is integrated into a coherent unit which may be installed in a measuring tube.
FIG. 1 shows the operating principle of these measuring systems corresponding to the related art. A fluid 112, for example, air, flows through a flow pipe 110 in an essentially laminar flow at a flow velocity vFL 114. Two ultrasonic transducers 116 and 118 are mounted on opposite sides of flow pipe 110 in such a way that first ultrasonic transducer 116 is able to emit ultrasonic waves, which may be received by second ultrasonic transducer 118, these ultrasonic waves propagating at a velocity vUL 120 at an angle α to flow velocity 114 which is different from 90°. In the system depicted here, the ultrasonic waves of ultrasonic transducer 116 propagate toward ultrasonic transducer 118 at a velocity vUL,1 which is higher than in an unmoving fluid 112 due to the motion of fluid 112 at velocity 114.vUL,1=vUL+vFL·cos α  (1)vUL stands for the propagation velocity of the ultrasonic waves in an unmoving fluid. In contrast, if ultrasonic waves are emitted by ultrasonic transducer 118 and received by ultrasonic transducer 116, these waves propagate at a velocity vUL,2 which is lower than propagation velocity vUL in unmoving fluid 112.vUL,2=vUL−vFL·cos a  (2)
Comparing a propagation time t1 which a signal needs from ultrasonic transducer 116 to ultrasonic transducer 118 with a propagation time t2 which an ultrasonic signal needs from ultrasonic transducer 118 to ultrasonic transducer 116 allows flow velocity vFL 114 of the fluid to be determined:
                              v          FL                =                              L                                          2                ·                cos                            ⁢                                                          ⁢              α                                ·                      (                                          1                                  t                  1                                            -                              1                                  t                  2                                                      )                                              (        3        )            A similar calculation of flow velocity vFL may also be performed for reflection systems such as described in EP 0 477 418 A1, for example.
The systems described in the related art, however, all have the problem that angle α in FIG. 1 must be sufficiently small for a successful flow measurement, but at least substantially smaller than 90°. This results in the problem that it is not possible to fit the surfaces of ultrasonic transducers 116, 118 flush to the inside surface of flow pipe 110. Protrusions 122 are thus formed in flow pipe 110 in the area of ultrasonic transducers 116, 118, which result in turbulences and flow separations. These turbulences cause pressure fluctuations and may result in interfering signal contributions which are superimposed on the actual ultrasonic signals as noise.
Another disadvantage of these turbulences and flow separations is that contaminants or particles such as dust, oil, or water droplets contained in the flowing medium tend to be deposited in the turbulence zones. One possible remedy is to insert wedge-shaped adaptor elements which fill up protrusions 122 of flow pipe 110 but are permeable to ultrasonic waves. The disadvantage here, however, is that the layer thickness of the wedge-shaped adapter elements varies over the cross section of an emitted ultrasound beam. This makes resonance adjustment for efficient ultrasound injection into the flowing medium difficult. Furthermore, such a construction responds sensitively to structure-borne noise injected into flow pipe 110.